1. Field of the Invention
The present invention relates to a novel molecular sieve composite membrane, more particularly to a composite membrane comprising an anodic alumina membrane as a templating support, as well as the vertically-oriented and closely-packed molecular sieve crystals grown in situ on anodic alumina membrane by a hydrothermal method, any pre- or post-treatments and modifications of substate is not required. This tailor-made synthesis is initiated by crystal growth in a geometrically confined environment, i.e., 200-nm-pore-diameter cylindrical channels of the support.
2. Description of the Prior Art
Crystalline zeolite and zeolite-like molecular sieves are aluminosilicate- and aluminophosphate-based materials with uniform intra-framework cages and channels of microporous or mesoporous size. With tailor-made acidity, specific pore structure and high thermal stability, they have been used as a unique type of catalysts for traditional, shape-selective chemical reaction of guest hydrocarbon molecules, especially in petroleum refining and petrochemical industry. Recently, attention has focused on the promising applications where molecular sieve crystals can serve as a potential nanometer or smaller sized host for the manufacture of advanced materials such as organized metal clusters, metal oxides or metal sulfides, conducting polymers, and trapped supra-molecular compounds that exhibit specific optic, optoelectronic and electrochemical properties and endow for devices of molecular wires, quantum electronics and nonlinear optics (Science, 263, 1698 (1994), Adv. Mater., 4, 612 (1992)).
To control exquisitely over chemical reactions and improve significantly the production of devices on the smallest scale, a technological challenge now is to be able to produce the zeolites or zeolite-like molecular sieves in the form of giant perfect single crystals or high quality thin films, rather than tiny powder forms prepared under conventional synthetic conditions. However, the synthesis of large single crystals is limited, the elaborate preparation of a continuous polycrystalline zeolite membranes has thus attracted much attention.
Much effort has been put into the preparation of molecular sieve membrane, for example: synthesis of composite zeolite films by embedding zeolite crystals in plastic materials (J. Membrane Sci., 73, 119 (1992)) and the in situ growth of zeolites on porous ceramic substrates by E. R. Geus et al. (J. Chem. Soc. Faraday Trans., 88, 3101 (1992), J. Membrane Sci., 82, 15 (1993), J. Chem. Soc., Chem. Commum., 339(1994), Microporous Mater. 3, 565 (1995)). The performance of these films is not quite satisfactory due to either the insufficient formation of zeolite crystals or the substrate having low porosity. Zeolite films with/without cellulose moulding were synthesized by T. Sano et al.(Zeolites, 11, 842 (1991), J. Chem. Soc., Chem. Commun., 2087 (1994)), these zeolite films were found to be either out of control in shape or lack of the mechanical strength.
Besides shape-selective catalysis and adsorption, the molecular sieve membranes can be used as hosts to orient guest atom cluster and molecules and can be considered as the promising template for sensor, conductor and non-linear optical materials. The advantage of preferred orientation of the guest molecules can be macroscopically improved by aligning the molecular sieve host crystals. Furthermore, it is also noteworthy that the use of this type of zeolite membranes on reaction and separation may have better performance than traditional zeolite powder and may lead to the new applicable industrial processes. However, preparation of a continuous film of controlled-orientation molecular sieve crystals on a porous substrate seems difficult. Increasing attempts have been made to develop aligned molecular sieve crystals, especially in a membrane form. For instance, pre-grown crystals of molecular sieves MFI and AFI were uni-directionally oriented by an electric field and then fixed by an epoxy resin film (Adv. Mater., 7, 711 (1995)); crystals of zinco-(or alumino-) phosphate molecular sieves were grown along(111) planes on gold surface that has been modified with organophosphate multilayer films (Science, 265, 1839 (1994), Nature, 368, 834 (1994)); crystals of MFI were observed to be ordered by mean of properly treated microstructured surfaces (Adv. Mater., 8, 137 (1996)); films of aligned mesoporous zeolite were prepared on the freshly cleaved mica(Nature, 379, 703(1996)) or at the air-water interface(Nature, 381, 589(1996)). Most of these methods needed the pre- or post-treatments and modifications of substrate.